to perform truncate log, you need the backup log permission:http://msdn.microsoft.com/en-us/library/ms179478.aspx
Security - Permissions
BACKUP DATABASE and BACKUP LOG permissions default to members of the sysadmin fixed server role and the db_owner and db_backupoperator fixed database roles.

however, truncating the log is not a regular action to be executed, only if really needed
typically, if the log file has grown MUCH too large, and you need to bring back the size to a reasonable size
in which case to not is that if you expect the log file to SHRINK as a result of the truncate log statement, you did not yet understand fully how the transaction log file works, and you should do some readings on that subject